Walmart is influenced by the ESRB ratings, in that they won't carry "AO" (Adult Only) games, fullstop. Much like the fact that they won't carry "X" movies, Walmart rightly puts a lot of value in the perception that they're a family-friendly retailer, concerned about children and the influence games may have on them.

Walmart is the biggest game retailer in the US, and they have a huge influence on what goes into games. Like the rest of mainstream America, they give all but the goriest violence a pass, but anything remotely suggestive of the existence of sex is forbidden. Which is fine: who am I to argue with the company responsible for 8% of all US retail non-auto sales? The point is that, for all practical purposes, pleasing Bentonville is a lot more important than pleasing the ESRB. Walmart does its own assessments, and makes its own demands of game publishers, and these carry huge weight with all game makers.